Complementary Metal-Oxide-Semiconductor (CMOS) image sensors (CIS) are used in numerous applications including digital still cameras. In semiconductor technologies, image sensors are used for sensing exposed lights emitted towards a semiconductor substrate. Generally, CIS products include a pixel (or pixel array) region and a periphery region. These products utilize an array of active pixels (i.e., image sensor elements or cells) including photodiodes and other elements (e.g., transistors) to convert images into digital data or electrical signals. Each of the photodiodes includes a p-type pinned photodiode and a n-type pinned photodiode to form a pn junction for transforming photons to electrons, and the performance of the photodiodes are affected by dark current (DC) or white pixel (WP) performance caused by various factors.
In a conventional skill, each of the photodiodes may include an n-type pinned photodiode and a p-type pinned photodiode. The n-type pinned photodiode may be formed in a p-type semiconductor substrate and then the p-type pinned photodiode is formed and embedded in a portion of the n-type pinned photodiode. The remaining portion of the n-type pinned photodiode is adjacent to the aforementioned transistor, and the aforementioned electrons may move through the remaining portion of the n-type pinned photodiode to a channel below and controlled by the transistor. However, the remaining portion has a relatively small area, which results in an additional potential barrier in the boundary between the p-type pinned photodiode and the transistor, and is disadvantageous to transferring the electrons and inducing an image lag accordingly. On the contrary, if the remaining portion has a relatively large area (which means the p-type pinned photodiode embedded in a relatively small area in the n-type pinned photodiode), the dark current or white pixel performance is poorer. Therefore, it is a trade-off between the image lag and the dark current or white pixel performance.